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Gold/Mining/Energy : Winspear Resources -- Ignore unavailable to you. Want to Upgrade?

To: russet who wrote (26812)	6/26/2002 12:15:19 AM
From: E. Charters	Read Replies (1) \| Respond to of 26850

I don't know where the pyrope or garnet curve overlaps the DSF exactly. Garnets may be found in metamorphic rock as well. Diamonds may be formed right on surface standard pressure, high temp, with a noble gas/hydrogen catalyst. It is just that it is rare to find that kind of temp without high pressure or too much oxidation, naturally. Garnets may be measured for their temperature by % nickel and by their pressure by the percent chromium roughly, arguing that they formed in the presence of chromite which may be a rough given in the chromite-harzburgite layer. Lamproites form a very shallow depths, perhaps 90 kliks, so the DSF may go from 300 kliks to that depth. Microdiamonds may be formed at any depths by some theories and may be unrelated to large diamond formation. As was said, the amount of chromium found in a garnet is not related too much to heat, but changes quickly with pressure, so it forms a barometer. The amount of nickel is sensitive mainly to heat, so it forms a thermometer of garnet formation. Plot one against the other and you have a pressure temperature curve of garnet formation. This precludes haveing to do inclusion intergrowth studies of widely separated rare rocks in the field. Good estimations of pressure and depth of a field can be done by selecting say 100 garnets from a kimberlite. Different kimberlites should be selected in the area as they may vary widely in age and origin, and original root keel depth. Some people cannot see how kimberlites that are right nearby each other may report to different controlling regional structures. In other words you may have a group of 30 to 160 million year old kimberlites reporting to NE faults, and adjacent some billion year old kimberlites reporting to some other direction of structure, or even radial faults. This is because the direction and hot spot origin of Pangea at the time of the different emplacement was markedly different for the two different ages. The continent was in a different place at different times, and moving in a different direction. So you can go ahead and assume a different controlling structure for widely spearate ages. Much has been made of NW rift valleys for controlling mineralization and emplacement. This is so only in a restricted sense. It is a area locus, not a temporal control of like mineralization. There is a difference. There is control is over time. Regional locii may be more or less the same time. Where the regional setting is disparate with drift direction, you may still find similar distal types as kimberlite fields are large, and even widely differing age types, but if you want to follow highly similar rocks, the historical continental drift direction is most promising. With thin crusts, you get relatively hotter temperatures at formation depth but faster cooling as you go up. Nickel thermometry depends on proton microprobe of nickel. My experience is with CRA labs so I cannot talk about other's experience. The main trouble is with establishing curves. You need the correlation info in lab. This technique can be done on any garnet and does not require pyrope. If a sample of garnets is taken from a kimberlite, for any group of garnets at a particular pressure, the one with the max temp is recorded. For all the pressures as the kimberlite ascends you have garnets forming at different temps; the max temp garnet is plotted. This gives the curve of max temp for each pressure, or the picture of the heat loss rate maximum for the crust. Lower watt outflow ratings show a thicker, cooler crust, more favourable for diamonds. EC<:-}